One of the main problems presently encountered in engine lubrication is due to the unavoidable presence in the lubricant of suspended extraneous particles such as carbonaceous matter and sludge originating from soot, products resulting from the deterioration of the motor-fuel and of the lubricant, and water.
The accumulation of these suspended matters raises a serious problem for lubricant efficiency in the engine and it is important to prevent the agglomeration and the deposition of these undesirable matters, as varnishes, hard carbonaceous matters and sludge, on the different parts of the engine. Since many years, attempts have been made to avoid these difficulties by using organo-metallic additives, such for example as sulfonates, phenolates or salicylates of alkaline-earth metals, or organic additives such for example as polymethacrylates, either grafted or copolymerized with nitrogen-containing monomers, or polyethylene-polyamine alkenylsuccinimides, or even polyol alkenylsuccinates.
However the use of organo-metallic additives is limited by the risk of deposition of various metal oxides on the spark plugs of spark-ignition engines; the deposits formed produce a pre-ignition detrimental to the engines. The known ashless organic additives suffer from the disadvantage of a limited efficiency at high temperature and also sometimes in the presence of water traces.
The European patent application EP-A-No. 72 645 discloses dispersing additives for lubricating oils prepared by a process comprising:
(1) reacting a polyalkenylsuccinic anhydride (for example polyisobutenylsuccinic anhydride whose polyisobutenyl chain has an average molecular weight by number from 900 to 2000) with an alkylene polyamine, and then PA0 (2) reacting the product from step (1) with an anhydride of dicarboxylic acid such as maleic anhydride, succinic anhydride and alkyl- and alkenyl-succinic anhydrides having from 1 to 18 carbon atoms in the alkyl or alkenyl chain. PA0 aliphatic polyols, preferably containing 3 to 6 hydroxyl groups, such for example as trimethylolpropane, pentaerythritol or dipentaerythritol; PA0 aliphatic aminoalcohols, preferably containing one primary amine group and 1 to 3 hydroxyl groups, such for example as 2-amino 2-methyl propanol or tris-hydroxymethyl aminomethane; PA0 polyamines of the general formula H.sub.2 N CH.sub.2 CH.sub.2 --NH CH.sub.2 CH.sub.2).sub.m NH.sub.2 as above defined ; and PA0 esters with free hydroxyl groups formed by reaction of an alkenylsuccinic anhydride with a polyol, such for example as the poly-isobutenylsuccinates of trimethylolpropane, pentaerythritol and dipentaerythritol.